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A-Site Disorder in Synthetic Fluor-Edenite, a Crystal-Structure Study

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A-Site Disorder in Synthetic Fluor-Edenite, a Crystal-Structure Study 21 The Canadian Mineralogist Vol. 32, pp. 21-30 (1994) A-SITE DISORDER IN SYNTHETIC FLUOR-EDENITE, A CRYSTAL-STRUCTURE STUDY KATHLEEN F. BOSCHMANN, PETER C. BURNS, FRANK C. HAWTHORNE, MATI RAUDSEPP1 AND ALLAN C. TURNOCK Depanmentof Geological Sciences, Universityof Manitoba, Winnipeg, Manitoba R3T 2N2 ABS1RACT Large crystals of amphibole were synthesized by slow cooling of a bulk composition nominally that of end-member fluor-edenite. Electron-microprobe analysis shows the crystals to be zoned from a core composition close to end-member fluor-edenite to a rim composition close to the fluor-tremolite - fluor-pargasite join. The crystal structure of a fragment of average composition Na0 2(Ca1.926Mg0.183)(Mg4.636Al0.364)(Si6.507Al1.493)022F2, a 9.821(1), b 17.934(4), c 5.282(1) A, ,9j � 105.08(1)0, V 898.3(1) A, C21m, Z = 2, was refmed to an R index of 2.9% for 1068 observed reflections collected with MoKcx X-radiation. Tetrahedrally coordinated Al is ordered at T(l), and octahedrally coordinated Al is ordered at M(2). The Na within the A-site cavity occupies both the A(m) and A(2) sites, and site-occupancy refmement shows Na to be equally partitioned between these two sites. This ordering scheme can be explained on the basis of local bond-valence requirements of the anions coordinating the T(1) site. Keywords: amphibole, fluor-edenite, synthesis, crystal-structure refinement, ordering, electron-microprobe analyses. SOMMAIRE Nous avons reussi a synthetiser de gros cristaux d'amphibole en refroidissant lentement un melange d'une composition globale egale a celle du pole fluor-edenite. Une analyse a Ia microsonde electronique demontre que ces cristaux sont zones, d'un coeur dont Ia composition est celle qui etait prevue a une bordure dont Ia composition est proche de Ia solution solide fluor-tremolite - fluor-pargasite. La structure cristalline d'un fragment dont Ia composition moyenne serait Nao.912(Ca1.926Mgo.Js3)(M�.636Alo.364)(Si6.507Al1.493)022F2, a 9.821(1), b 17.934(4), c 5.282(1) A, � 105.08(1)0, V 898.3(1) 3 A , C2/m, Z = 2, a ete affinee jusqu'a un residu R de 2.9% en utilisant 1068 reflexions observees (rayonnement MoKcx). L'aluminium a coordinence tetraedrique occupe T(1), tandis que !'aluminium a coordinence octaedrique occupe M(2). Le Na du site A occupe les sites A(m) et A(2); l'affmement indique une repartition egale de l'atome Na entre ces deux sites. Ce schema de mise en ordre decoule des exigeances locales des valences de liaisons des anions en coordinence avec T(l). (Tradnit par Ia Redaction) Mots-cles: amphibole, fluor-edenite, synthese, affinement de Ia structure cristalline, mise en ordre, donnees de microsonde electronique. INTRODUCTION crystals. Na et al. (1986) reported the coexistence of diopside and sodian phlogopite up to 3 kbar within the There have been several attempts to synthesize field of edenite - richterite (- tremolite) solid solution, edenite, ideally NaCa2Mg5Si7Al022(0H)2, but none of and the results of Raudsepp et al. (1991) are broadly themhave been very successful. Widmark (1974) and compatible with this. Thus the edenite end-member Hinrichsen & Schi.irmann (1977) both reported the has not been synthesized as yet, and there is a strong synthesis of edenite, but the run yields were low, and possibility that it may not be stable. On the other hand, Greenwood (1979) concluded that Widmark (1974) fluor-edenite has been synthesized on end-member had synthesized overgrowths of tremolite on his seed composition (Graham & Navrotsky 1986). Raudsepp 'Present address: Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4. 22 TilE CANADIAN MINERALOGIST et al. (1991) attempted to synthesize large crystals of fluor-edenite by slow cooling of the nominal bulk composition from above the liquidus. Although elec­ tron-microprobe analysis showed that the crystals depart from the ideal fluor-edenite composition, the crystals were sufficiently large to characterize by crystal-structure refinement; the results are presented here. EXPERIMENTAL Synthesis Forty mg of starting materials (fused silica glass, y-Al203, CaF2, Na2C03, MgO) were sealed in a -1A-- 4 x 23 mm Pt tube and placed in a conventional verti­ cal quench-furnace. The run was held at 1240°C for 1 h and then cooled to 816°C over a period of 332 h. Further experimental details are given by Raudsepp et al. (1991). In general, the run products were >90% amphibole, with minor forsterite, plagioclase, clinopyroxene and cristobalite. One particular run gave one large crystal (-10 x 3 mm) with minor fine-grained non-amphibole phases; this crystal was gently crushed to give more conveuiently sized fragments, one of which was used in thepresent study. Collection of the X-raydata An irregular lath-like fragment of synthetic fluor­ edenite was mounted on a Nicolet R3m automated four-circle diffractometer. Twenty-two reflections were centered using graphite-monochromated MoKa X-radiation. The unit-cell dimensions (Table 1) were derived from the setting angles of twenty-two auto­ matically aligned reflectionsby least-squares methods. A total of 1435 reflections was measured over the range (3° < 29 < 60°) with index ranges 0 � h � 13, 0 � k � 25, -7 � l � 7; reflections forbidden by the C face-centering were omitted. Two standard reflections were measured every fifty reflections; no significant changes in their net intensities occurred during data TABLE 1. MISCELLANEOUS INFORMATION FOR FLUOR-EDENITE Space group C2/m Crystal size (mm) 0.1Ox0.24x0. 1 0 FIG. 1. Difference-Fourier sections throughthe centralA(2/m) site with the A cation removed from the structural model: a(A) 9.821(1) Total Ref. 1435 (a) (100) section at x = 0; (b) (010) section at y = \12; (c) b!Al 17.934(4) [l(obsl > 2.5a(l)) 1068 section through the electron-density maxima on the 2-fold c(A) 5.282(1) axis and in the mirror plane, approximately parallel to Pl0l 105.08(1) Final Rlobs) 2.9% (201).In (a) and (b), the contour interval is 1 e/A3• In (c), VIA3l 898.3(1) Final wR!obs) 3.0% the contour interval is 1 e/A3 below 6 e!A3, and 0.1 e!A3 above 6 e/M; only this close contouring at high levels of Ideal cell contents: 2(NaCsaMg,Si,AIO,.F2l density brings out the separate maxima on the 2-fold axis cell contents: Analyzed 2!Na..812Cs,..,.Mo..so1Aio.-1Si�..,AI1,492)022F21 and within the mirror plane, and emphasizes the minimum R = IIIFoi·IFcl}f IIFol at thecentral (0 \12 0) position;this minimum is dot-shaded wR = !Iw!JFoJ·JFcJl'IIwf'!I"'· w=1 to distinguish it fromthe surrounding maxima. A-SITE DISORDER IN FLUOR-EDENITE 23 collection. An empirical absorption-correction based TABLE 2. ANAL POSITIONAL PARAMifrERS AND EQUIVALENT ISOTROPIC DISPLACEMENi PARAMETERS on 396 psi-scan intensities was applied, reducing R (.A2x104) FOR FLUOR-EDENITI: (azimuthal) from 1.3 to 0.9%. The data were corrected for Lorentz, polarization and background effects, and X y 11 u..,... reduced to structure factors; of the 1435 reflections n1l 0.28176(8) 0.08479(4) 0.3025(2) 82(2) measured, there were 1068 unique observed [I � 2.5o(I)] reflections. n2l 0.29057(8) 0.17251(4) 0.8113(2) 71(2} M(1) 0 0.08902(7) 1/2 66(4) Structure refinement M(2) 0 0.17547(7) 0 64(4) M(3) 0 0 0 66(6) Scattering curves for neutral atoms, together with M(4) 0 0.27854(5) 1/2 115(3) anomalous dispersion corrections, were taken from Cromer & Mann (1968) and Cromer & Liberman A(m)** 0.0415(9) 1/2 0.0953(20) 200* (1970), respectively. The Siemens SHELXTL PLUS A(2)** 0 0.4765(5) 0 200* (PC Version) system of programs was used throughout 0(1) 0.1084(2) 0.0860(1) 0.2182(4} 97(5) this study. R indices are of the form given in Table 1, 0(2) 0.1191(2) 0.1714(1) 0.7312(4) 91(5) and are expressed as percentages. Least-squares refinement of the structure in the 0(3) 0.1044(3) 0 0.7133(5} 107(7) space group C2/m, with starting parameters taken from 0(4) 0.3656(2) 0.2504(1) 0.7884(4) 115(6) Hawthorne (1983) and an isotropic displacement 0(5) 0.3511 (2) 0.1391(1) 0.1108(4) 128(6) model, converged to an R index of 5.1%. Conversion 0(6) 0.3462(2) 0.1164(1) 0.6075(4) 133(6) to an anisotropic displacement model, together with the refinement of all parameters, converged to a 0(7) 0.3448(3) 0 0.2784(7) 129(8) wR R index of 3.2% and a index of 3.3%. A model . with weighted structure-factors and an isotropic fixed during refinement • * A(ml and A(2) populations are 0.24(1 land 0.25(1l Na, extinction correction was tried but did not lead to any respectively. improvement in the refinement. As is usual for amphi­ boles with a fullA -site, the A(2/m) displacement factor was found to be very large (U "' 0.16), indicative of significant positional disorder in the cavity sur­ TABLE 3. ANISOTROPIC DISPLACEMENT PARAMETERS CA"x104) rounding theA site (Hawthorne & Grundy 1972). This FOR FLUOR-EDENITE feature is of considerable interest in the present case u, u22 u.. u.. u,. u,. because of the fairly simple composition of the amphi­ 78(4) 84(3) 80(3) -2(3) 14(3) -3(2) bole.
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